This invention relates to a device for measuring torsional torque applied to a fixed or revolving shaft, which is made of a magnetic material at least in its surface layer, by utilizing a magnetostrictive phenomenon.
It is well known to measure torsional torque applied to a fixed or revolving shaft by a magnetic method utilizing the following phenomena.
The torque produces greater strains in the surface region of the shaft than in the core region, and the torque produces a tensile stress around the shaft surface in a direction inclining to the right or left, depending on the direction of the torque, at an angle of 45.degree. with the direction of the longitudinal axis of the shaft and a compressive stress in the reverse direction at an angle of 45.degree. with the axial direction. When the shaft material is a magnetic material the tensile and compressive stresses cause the magnetic material to exhibit a magnetostrictive effect and induce changes in permeability. That is, the permeability increases in the direction of the tensile stress when the magnetostriction of the shaft material is of the positive type and decreases in the same direction when the magnetostriction is of the negative type.
Torque measuring devices using the above phenomena have at least one exciting coil to produce a magnetic circuit around the shaft to which torque is applied and means to detect a change or changes in permeability of the shaft material.
Conventional torque measuring devices are not yet fully satisfactory in sensitivity and/or accuracy particularly when applied to revolving shafts. The reasons for insufficient accuracy of torque measurement include uneven and locally anisotropic distribution of the permeability of the shaft subject to measurement.
Some proposals have been made for improvements in sensitivity and accuracy of torque measurement by the magnetic method. For example, JP-A No. 59-77326 shows a torque measuring device including a film of a magnetostrictive material such as an iron based amorphous alloy, which is formed with two series of parallel slits and is wrapped around and bonded to the shaft to which torque is applied. On the shaft surface the first series of parallel slits in the film incline at an angle of about 45.degree. with the direction of longitudinal axis of the shaft, and the second series of parallel slits reversely incline at an angle of about 45.degree. with the axial direction. The two series of slits are symmetrical with respect to a plane cross-sectional of the shaft. The device has two exciting coils positioned around the two series of slits, respectively. When torque is applied to the shaft the magnetostrictive film undergoes tensile deformation along one series of parallel slits accompanied by a considerable increase in permeability and compressive deformation along the other series of parallel slits accompanied by a considerable decrease in permeability. Such changes in permeability are detected by a bridge circuit. However, it is a disadvantage of this device that an indispensable adhesive layer between the shaft surface and the slitted magnetostrictive film tends to deteriorate as torque is repeatedly applied to the shaft, as the shaft temperature changes and/or as time passes. Deterioration of the adhesive layer will result in changes in the relationship between the magnitude of torque applied to the shaft and the amount of deformation of the magnetostrictive film and, therefore, in lowering of the accuracy of torque measurement.